check if instance manager not found on delete, don't return an error

[megan07]

Fixes hashicorp/terraform-provider-google#14516

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

compute: fixed crash on `terraform destroy -refresh=false` for instance group managers with `wait_for_instances = "true"` if the instance group manager was not found

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

Terraform GA: Diff ( 2 files changed, 24 insertions(+))
Terraform Beta: Diff ( 2 files changed, 24 insertions(+))
TF Conversion: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[modular-magician]

Tests analytics

Total tests: 2719
Passed tests 2433
Skipped tests: 279
Affected tests: 7

Action taken

Found 7 affected test(s) by replaying old test recordings. Starting RECORDING based on the most recent commit. Click here to see the affected tests

TestAccComputeFirewallPolicyRule_multipleRules|TestAccAlloydbBackup_missingLocation|TestAccAlloydbCluster_missingLocation|TestAccApigeeKeystoresAliasesKeyCertFile_apigeeKeystoresAliasesKeyCertFileTestExample|TestAccApigeeKeystoresAliasesPkcs12_ApigeeKeystoresAliasesPkcs12Example|TestAccDataSourceAlloydbLocations_basic|TestAccDataSourceGoogleFirebaseAndroidAppConfig

Get to know how VCR tests work

[modular-magician]

Tests passed during RECORDING mode:
TestAccAlloydbBackup_missingLocation[Debug log]
TestAccAlloydbCluster_missingLocation[Debug log]
TestAccApigeeKeystoresAliasesKeyCertFile_apigeeKeystoresAliasesKeyCertFileTestExample[Debug log]
TestAccApigeeKeystoresAliasesPkcs12_ApigeeKeystoresAliasesPkcs12Example[Debug log]
TestAccDataSourceAlloydbLocations_basic[Debug log]
TestAccDataSourceGoogleFirebaseAndroidAppConfig[Debug log]

Tests failed during RECORDING mode:
TestAccComputeFirewallPolicyRule_multipleRules[Error message] [Debug log]

Please fix these to complete your PR
View the build log or the debug log for each test